In general, a mobile communication system was developed for the purpose of providing a voice service while securing user's mobility. However, the area of the mobile communication system has been expanded gradually to data services in addition to voice services, and, at present, the mobile communication system has been developed to the extent of being capable of providing high-speed data services.
Nowadays, in the mobile communication system, there has been a need to solve a resource shortage problem due to providing of various services and to provide a greatly developed mobile communication system in accordance with a user demand to receive higher-speed services.
To cope with such a need, standardization of a Long Term Evolution (LTE) system in the 3rd Generation Partnership Project (3GPP) is currently underway as one of the next-generation of mobile communication systems that are being developed. The LTE system is a technology to implement high-speed packet-based communication having a maximum transmission speed of about 100 Mbps. For this, there have been discussions on several schemes; for example, a scheme to reduce the number of nodes that are located on communication paths through simplification of the structure of a network and a scheme to make radio protocols maximally approach radio channels.
On the other hand, in order to support a voice call in an LTE network, Packet Switched (PS) type IMS-based Voice over LTE (VoLTE) technology may be used, or CS Fall Back (CSFB) technology that reuses a Circuit Switched (CS) type of a 2G/3D system may be used. In the LTE network, the term “VoLTE” is a wording that can be used in the same concept as Voice over IMS (VoIMS). Further, in the LTE network, an emergency call may be serviced through the VoLTE.
The current LTE system may be constructed to accept an emergency call service or a VoLTE service in a partial area of the network and to non-accept the emergency call service or the VoLTE service in the remaining area. In this case, a VoLTE voice call is connected with respect to User Equipment (UE) in the VoLTE service acceptance area, and if the UE moves to the VoLTE service non-acceptance area, the ongoing VoLTE voice call is disconnected when a new voice call arrives in a CS domain even if the VoLTE voice call is maintained. In the same manner, an emergency call is connected with respect to the UE in the emergency call service acceptance area, and if the UE moves to the emergency call service non-acceptance area, the ongoing emergency call is disconnected when the voice call arrives in the CS domain even if the emergency call is maintained. Since such a voice call disconnection phenomenon may cause severe problems, there is a need to make researches in a method that can solve the voice call disconnection problems.